There are over 80,000 species of flies and mosquitoes and these as a group vector more diseases than any other order. Characteristics of the order include one pair of wings, halters (knoblike reduced second wings), and complete metamorphosis. The adults vector diseases mechanically and biologically, while the larvae often invade tissue of higher animals. Members of this order have a wide range of breeding habitats including plants, flesh, excrement, and standing water.
Myiasis is the condition where fly maggots infest organs and tissues of living humans or other animals. There are several types depending which part of the body is infested and how it occurred. The more common types include the following: enteric (both intestinal and gastric), urinary, nasal, auricular (ear), ophthalmic (eyes) and dermal/cutaneous. When wounds are involved it is referred to as traumatic. Accidental myiasis occurs due to ingestion of maggots by consuming infested food. This can also occur when the maggots invade an anal opening. Semi-obligate myiasis occurs when the maggots invade dead tissue and progress into live tissue that surrounds a wound. Obligate myiasis is when the maggots only infest living tissue.
Flesh Flies and Blow Flies
The flies most frequently associated with traumatic myiasis are the flesh flies and blow flies. Blow flies are often found in houses and have a characteristic green, copper or blue metallic coloration. The larvae feed on dead animals, excrement, and occasionally living flesh.
A typical blow fly
The flesh fly is found near carrion, excrement, and decaying organic matter. It has a characteristic checkerboard pattern to the abdomen and has 3 dark stripes on the thorax. These flies can get quite large. Instead of laying eggs, flesh flies give birth to live maggots, which are either deposited or dropped mid-air. I was once standing in a motel room in Costa Rica and a flesh fly deposited a larva on my arm. Within less than a minute the larvae began to attempt to bore into my skin.
A typical flesh fly. Image courtesy of Marcelo de Campos Pereira, University Sao Paulo, Brazil.
In the United States most cases of traumatic myiasis are more or less confined to rotting tissues. Typically these flies deposit their larvae or eggs (hit) that hatch and feed chiefly on the decaying tissue associated with wounds. This feeding activity rarely harms the person or animal, and in some cases can actually be beneficial. It was found during World War I that wounded troops not found immediately but left in the battlefield for a few days developed less of certain types of deep-seated infections than those soldiers taken directly to the hospital. The wounds of the soldiers left in the field were infested with fly maggots, which secreted a material named alantoin, a natural antibiotic. The introduction of blow fly maggots into wounds is still practiced worldwide by many primitive people and the use of sterilized maggots for this purpose is now receiving some attention by physicians in the United States.
Under certain conditions this type of infestation can be dangerous, if not deadly, to animals. Of course, this is partially due to the inability of many animals to clean infested wounds. For example, we once had a rabbit that accidentally dipped its dewlap (a large flap of skin under the chin) into its drinking water every time it drank. As a result some of this tissue became excessively damp and began to rot. A blowfly made a hit of several dozen eggs and by the time we discovered it, the maggots were feeding on the entire underside of the still living animal. Of course the animal had to be destroyed.
One example of semi-obligatory myiasis is the screwworm fly, Cochliomyia hominovorax, which is attracted to the wounds/sores of cattle and other animals. The eggs are laid in batches of 200 to 500 in 2 to 10 day-old wounds and hatch after 14 to 18 hours. The larvae initially feed of rotting tissue but quickly bore into healthy flesh and feed for 3 to 5 days. Once fully developed the larvae drop to the ground to pupate and after 7 days emerge as adults. Wounds infested with these flies become extensive, attracting other flies and even more of the screwworm flies. In some cases the infested animal becomes weakened, may not feed or drink and, if not treated, may die. This pest is no longer found in the United States because of a successful sterile male release program. This blowfly is still a major pest of cattle, ranging from Mexico to South America.
The adult of a primary screwworm, a major cattle pest from Mexico to South America. Image courtesy Marcelo de Campos Pereira, Sao Paolo, Brazil.
In many areas of the world human myiasis from this fly is fairly common. In 1935, prior to its eradication in the United States, over a hundred cases of human myiasis occurred in Texas alone. In one documented case, a female fly deposited her eggs up the nostril of a man who had a cold. Apparently, the fly was attracted to his nasal discharge. The first symptoms were those of a severe cold. As the larvae cut away through the various tissues of the head, the victim became slightly delirious and complained of intense misery and discomfort in the nose and head. When the larvae finally cut through the soft pallet, his speech was impaired. Despite attempts to remove the larvae the patient, after a short recovery, had a relapse as the Eustachian tubes were invaded. The tissue damage was extensive and the head and face showed the characteristic swelling of screwworm myiasis. During the autopsy over one hundred larvae were removed from the man’s brain.
The house fly, Musca domestica, is one of the most well known insect pests in the world. This fly has 4 longitudinal stripes on the thorax (Figure 87). The eggs are laid in batches of 100-150, with a female capable of producing over 1,000 in her lifetime. The eggs hatch in about 8-12 hours and the larvae are worm-like with no visible head area. The larvae or maggots go through 3 successive molts that, under optimum conditions, take 2 to 5 days. With many flies the outer skin of the last instar larva hardens prior to pupation and forms a capsule like structure within which is found the pupae. The puparium is dark and will take 3 to 4 days to develop into the adult fly. Under ideal conditions the house fly can complete one life cycle in as little as 7 days. With this number of eggs and speed of development these insects have a tremendous reproductive capacity. Someone once figured out that if one female house fly in April laid all her eggs and all her offspring survived and reproduced similarly, by August there would be 191,0100,000,000,000,000,000 flies or enough to cover the earth by 43 feet. This obviously doesn’t happen. The limiting factor is the availability of food. Of course, in the United States we have health codes that, when followed, greatly reduce the breeding sites of these pests.
The common house fly, Musca domestica.
The adult lives for about 30 days and prefers to oviposite on decomposing manure or vegetable matter. Normally most fly infestations around the home are of local origin; however, these flies are capable of flying up to 20 miles if need be.
The house fly occasionally causes enteric myiasis. One interesting case resulted when an elderly lady went to the doctor complaining of intestinal and stomach cramping. After considerable consultation, a stool sample revealed living house fly maggots and pupae. Apparently she had consumed some food that was infested with maggots. These maggots passed through the digestive system and were basically impervious to digestive enzymes and other digestive processes. Normally under these conditions maggots in the digestive system may survive but do not complete their life cycles. Rectal myiasis has also resulted when house fly adults lay their eggs around the anus. Upon hatching the maggot migrate up into the rectum causing considerable discomfort. (So, wipe well!)
The most important medical implication of the house fly is its potential to vector disease causing organisms. The house fly freely enters the home, restaurants and other places where human food is available; it just as freely inhabits situations where human and animal excrement is available. It feeds on human food and excrement! Because the fly can only feed on liquids it regurgitates digestive enzymes from its stomach to dissolve solids. In doing so it may also regurgitate drops of excrement. The fly is also structurally adapted for picking up pathogens. Its mouthparts are provided with many fine hairs and ridges that readily collect germs and filth. The tarsi are a complex structure of fine hairs and sticky pads that enhances it potential for vectoring diseases. Studies have indicated that a single house fly can carry as many as 6 million bacteria on its body. Flies have been known to be contaminated with more than 100 species of pathogenic organisms, including the causative agents of amoebic and bacterial dysentery, typhoid fever, cholera, salmonella, anthrax, leprosy, yaws, trachoma, polio, and infectious hepatitis. In addition they have been demonstrated to carry the eggs of certain pathogenic worms including pinworm, tapeworms and hookworms.
Lesser House Fly
The lesser house fly, Fannia canicularis, commonly inhabits houses as well but is found all over, not just primarily in the kitchen, as is the house fly. The larvae are quite small and have spines covering the body and are usually found in excrement and decomposing organic matter. These flies can be readily distinguished from the house fly by their flight pattern of hovering or flying in circles in garages and breezeways. They can vector many of the same diseases as the house fly.
The lesser house fly, a more common inhabitant of homes than the house fly.
The stable fly Stomoxys calcitrans, is a blood feeder with piercing-sucking mouthparts. This fly has the same 4 stripes as the house fly but, as indicated, has bayonet-shaped mouthparts that project forward from the head. It is sometimes referred to as the biting house fly as it readily enters the home. This is true especially in the autumn during periods of rainy weather. This species attacks many types of animals but can become especially bothersome to dogs in the summer and fall months. It has the nasty habit of feeding on the nose of dogs. This can readily be recognized by the many blood spots on this area. It is also commonly found around livestock upon which they feed with a painful bite and are therefore a tremendous annoyance to cattle. Some animals react to their presence with decreased milk production.
An adult stable fly with bayonet shaped mouthparts. Image Courtesy of Whitney Crenshaw, Univ. Georgia-Bugwood.
These flies breed in manure mixed with hay or damp lawn cuttings. The eggs resemble those of the house fly and are laid in clutches of 25-50. The larvae pupate in dry areas with a total lifecycle of 28 days. The lifespan of these flies is 60-70 days.
The tsetse fly, Glossina spp., is well known for inhabiting tropical and sub-tropical areas of the world. Both sexes are blood feeders and attack a wide variety of hosts. This fly is approximately 2 times the size of the house fly and is brownish in color. It feeds in the daylight and is attracted to movement. The female fly gives birth to fully grown larvae, having passed 3 larval instars inside the female’s body where they feed on a specialized milk gland. Once released, the larva immediately pupates and develops into adults in 3 to 4 weeks.These flies vector African sleeping sickness (Trypanosoma brucei), which is related to Chagas Disease. This organism invades the cerebral spinal fluid and various organs of the body. It is also a major problem in cattle and has prevented the development of this industry in certain parts of Africa. The disease kills over 7,000 people per year. First phase symptoms of the disease include irregular fever, enlarged glands in the neck and slow debilitation. The second phase includes increased enlargement of the lymph glands and jerking movements. The disease may progress into coma and eventual death. There are strains of the trypanosome with the ’Gambian’ strain being less severe and the ’Rhodesian’ strain.
The disease is transmitted by the pathogen adhering to the mouthparts of the fly and completing part of the life cycle inside the gut of the insect. The pathogens then move to the salivary glands of the insect and are injected into the new host. G. palpali, is the principle vector of the ’Gambien’ strain and inhabits the heavy forest of West Africa, commonly known as the "Congo." G. morsitan, vectors the ’Rhodesian’ strain of the disease and is found in the Savannahs of Eastern Africa. This fly prefers big game animals and humans. Various control strategies are employed such as traps, insecticide applications, natural enemies, sterile male releases, and modification of the environment. None have proven to be very effective. Drug therapy, although highly toxic, is utilized as well as antibiotics. This disease is considered by the World Health Organization (WHO) to be an epidemic in Central Africa with 20,000 new cases being reported a year.
The eye gnats, Hippelates collusor and H. pusio, are small flies (slightly larger than a grain of sand) that are attracted to mucous secretions of the eye. These flies do not bite, but persistently feed on eye secretion. They approach their mammalian hosts, commonly including humans, quietly alighting some distance from their feeding site. To reach this site they crawl over the skin or take short repeated flights thus adding to the host’s annoyance. I’m getting annoyed just thinking about it! The eye gnats breed in decaying vegetation, manure, and loose sandy soil. They have been implicated in the spreading of "pink eye" (conjunctivitis). They are quite common throughout much of the southern United States ranging from California to Florida. They are found in high numbers in the Coachella Valley of Southern California and prefer hot temperatures.
An eye gnat commonly attracted to the eyes especially in hot weather.
The cattle grubs are fairly common pests of cattle and other large mammals. There are 2 major species of cattle grubs in the United States, namely the northern and common cattle grubs. These are also known as the heel flies, bomb flies or ox warbles. Anyone who works with cattle is undoubtedly familiar with the large tumor like swellings that develop on the backs of these animals during the winter and early spring months. If this tumor is squeezed, a large grub-like maggot, about the size of an elongated marble, will pop out.
A common cattle grub occurring throughout much of North and South America. Image courtesy Marcelo de Campos Pereira, University of San Paolo, Brazil.
The adult flies deposit their eggs on the hairs in the lower legs and heel. They are not very secretive and frequently buzz loudly in doing so: hence the name bomb fly. There is no real pain to the cow during this oviposition process but because of the loud and aggressive nature of this attack the cattle become terror stricken and gallop madly for water or shade. Once hatched, the larvae bore into the skin and internally travel up the legs through the tissues, spend a time in the stomach and finally end up lodging beneath the skin of the back where it forms large cysts. Soon after reaching the back it cuts a small breathing hole through the skin. Once fully grown, the mature larvae eat their way through the skin and drop to the soil to pupate. The entire life cycle takes about a year to complete.
Damage from these insects is multifold. The injury is first irritation caused by the migrating larvae through the animal’s body. This typically results in weight loss. When the maggots emerge from the back the hide is cut and potential leather ruined. The sore from these wounds may fester and secondary infection occurs. One major problem is the animal’s reaction to fly oviposition. Animals are frequently hurt in their attempts to escape. Occasionally a cow will rush off a high cliff in its attempt to escape. Pregnant females have been known to abort.
This fly can also infest humans with symptoms including itching, pain, and cramps and possible blindness or death with the larvae ending up in the chest, neck, brain, spine, and eyes. The following is a narrative of a human infestation. “Several days after initial infestation, exact time not remembered, soreness was experienced and a slight swelling in the region of the right groin appeared. In about a week the swelling had increased to the width of a hand with no discoloration. The swelling then crept downward toward the left side affecting the scrotum, thence downward along the left leg to the knee and calf, thence back up the left leg following about the same course to the left groin, thence across to the right groin and back again to the left and upward along the left side of the body, slightly anterior to the shoulder, thence downward to the upper right arm to near the elbow, when the arm could not be raised without great pain, thence the swelling traveled upward again to the neighborhood of the shoulder blade where a “hive-like” local swelling was formed without any itching sensation. Mr. C. stated that at this point he was “bothered” all night, and while rubbing his arm and manipulating his shoulder muscles a larva of some insect “popped out”.
Human Bot Fly
The human bot fly, Dermatobia hominis, is a pest of mammals, birds, and humans and is common in Mexico, Central and South America. The larvae are known as a variety of name including torceli, torsalo and berne. It is a major pest in Brazil and Central America, where young, heavily infested animals may be killed and in cattle the loss of meat, hides and milk has at times severely crippled this industry. In humans, various parts of the body can be infested, including arms, legs, back, scrotum and buttocks.
An adult human bot fly. Image courtesy Marcelo de Campos Pereira, University of Sau Paolo, Brazil.
The life history of this fly is quite unique. The females does not deposit her eggs on the primary host, but seeks out and captures a mosquito, fly or other arthropod and glues her eggs on this critter (Figure 93). The main carrier is a large day-flying species of mosquito. Over 48 species of other flies (mosquitoes, black flies, deer flies and others) and one species of tick have also been implicated in this cycle. The eggs are attached to the carrier so the top end points downward. Then when the mosquito makes contact with a potential host, the larva can immediately emerge and is in optimum position for attachment. When the mosquito begins to feed on its host the fly larvae emerges from the eggs and bores into the skin, penetrating the subcutaneous areas. The larval period in the body is about 6 weeks--at the end of which it bores out, drops to the ground and pupates.
Human Bot Fly
A common fly carrying the eggs of the human bot fly. Image courtesy of University of Sau Paulo, Brazil.
One scientist allowed 2 human bots to enter his skin and reported the following. “I allowed 2 larvae to bore into the skin on my arm. The first required 42 minutes and the second 1 hour and 35 minutes. I felt no sensation for the majority of the time but as the larvae were disappearing below the skin I felt a sharp pricking. At first there was a sharp itching at night but within a few days lesions develop which looked like boils and by the end of 3 weeks they were excruciatingly painful. After about 50 days the large larvae emerged and dropped from the skin. No pain was felt at the time of emergence”.
Blood Sicking Maggots
Blood sucking maggots are even more disgusting than the human bot fly. These occur throughout much of Africa and are commonly found in homes. The eggs are deposited in various situations, such as sleeping mats on the ground, in cracks and crevices and other situations where the larvae can find food when they hatch. The larvae are remarkably resistant to drying out. They are nocturnal, sucking the blood of sleeping people. They produce a wound with their powerful mouth hooks and feed nightly for 15 to 20 minutes when a host is available. The bite of the maggot is felt as little more than a pinprick. The effect of the bite is quite variable depending on the individual’s sensitivity and their attitude about blood sucking maggots.
Horse Flies and Deer Flies
The horse flies, Tabanus spp., and the deer flies, Chrysops spp., are cosmopolitan in distribution with deer flies typically found at higher elevations. Both types attack large mammals including humans. Only the female is a blood feeder with the male feeding primarily on nectar. The bite of these flies is painful, due to the lacerating mouthparts. The fly creates a wound and then laps up the blood. Although mainly a nuisance to humans, large animals lack the ability to avoid the bites of these persistent creatures. Under certain conditions the loss of blood due to consistent feeding by a large number of flies can become a major problem. Scientists have recorded over 1,000 horse flies feeding on a single cow over an 8-hour period. The amount of blood loss in this situation would amount to a little over a gallon. These flies have been known to vector anthrax (Bacillus anthracis) as well as tularemia (Francisella tularensis). Anthrax can be mild if only lesions form on the skin, but can be serious if the spores are inhaled or ingested. Tularemia causes a high fever, swollen lymph glands, septicemia, and lung infection.
An adult horse fly
An adult deer fly
Black flies Simulium spp. and others, are very small insects that have blade-like mouthparts that slash and lap blood much like horse and deer flies. These day flying flies are black, thick bodied, and have a prominent "hump" on the thorax. The larvae are found in running water and are attached to rocks and vegetation via silken anchor threads. The larvae pupate in cocoons under water from which the adults eventually emerge. These flies are found in the US and Canada and have caused large amounts of damage and death to livestock and in certain areas are an extreme problem to humans. The bite of these day-flying insects is initially benign. However within hours small hard red bumps develop and the intense itching begins which can last up to a week or more. Some victims have allergic reactions. Inhalation of the fly by livestock causes death.
Some species of these flies vector several serious diseases, including African river blindness and South American Robles disease. These diseases are caused by a filarial roundworm that is found underneath the skin of the victim. Lesions form in the tissues that become quite large and full of worms. The itch from these lesions is severe and has led to suicide in some victims. The young worms can also migrate into the eyes and cause blindness. This disease is very serious with over 80,000 people suffering from blindness in Africa. Spraying of insecticides does control the black fly. Treatment includes the medication Ivermectin that interrupts the reproductive lifecycle of the parasite.
Phlebotimine Sand Flies
These flies resemble moths in that they are covered in hairs and are 1.4 - 4 mm long. The females are nocturnal blood feeders that attack mammals, birds and reptiles. The larvae feed on dead and decaying plant matter in dark damp locations.
They are important vectors of several severe diseases. Oroyo fever or Carrion’s disease (Bartonella bacillafarmis) is frequently fatal and causes severe anemia. Sand fly fever or Pappataci Fever is caused by a virus and has a duration of only 3 days and is usually non-fatal. It is found in the Mediterranean, China, and India.
Leishmaniasis (Leshmania spp.) is a protozoan-caused condition that has infected over 12 million people of the tropics and sub-tropics. There are various types of this infection that attack different areas of the body. "Kala-azar" affects the viscera and the skin becomes very gray in color. "Oriental Sore" is found in the subcutaneous layers of skin and is located in Europe, Africa, and Asia. "American Leishmaniasis" is very disfiguring and destroys the cartilage and bone of the nose and mouth. This is found in South America and primarily in Brazil. These diseases are second only to Malaria in seriousness. There are various drugs available to treat the disease, but which are considered very toxic.
No-Seeums and Punkies
These are very tiny, blood sucking flies that can be troublesome to humans. The name no-seeum refers to the fact that the smaller forms can pass through the holes in a window screen and are difficult to see. The larval stage of these flies is typically semi-aquatic to aquatic or at least they breed in moist soil; common breeding sites include salt or fresh water, tree holes, decaying plant material such as cactus, banana leaves and moist sandy soil.
These tiny flies become major problems in recreational areas, especially in coastal areas around fresh water inlets and tidewater pools, and in mountain areas, where they become so common that they drive tourists out. The bites of this blood-sucking flies cause itching in sensitive individuals and welt and lesions that may persist for up to a week or more.
We once took a student group down to San Blas, a costal jungle community along the Coast of Mexico. One of the participants drank several beers before retiring (passing out) on an outdoor hammock wearing nothing but a swimming suit. When he awakened the next day the punkies had had a field day. His entire body was covered with the bites of these flies, an average of 13 per square inch. To say the least he was pretty miserable for several days. These flies can vector pathogenic nematodes, protozoans, and viruses. Control includes painting the screens with insecticides and draining their breeding sites.
Blowflies are “metallic” in coloration and in the maggot or larval stage mostly feed on decaying flesh.
The screw -worm begins by feeding on decaying flesh but quickly moves into healthy flesh making it a serious pest of cattle and other animals in “Mexico to South America”.
The infestation of a living animal with fly maggots is called myiasis. In some situations with some flies this could be considered beneficial to the host animal.
Studies have indicated that a single house fly can carry as many as ”16 million bacteria” on its body.